Radar device and a method for suppressing interference in a radar device

ABSTRACT

A radar device includes elements ( 10 ) for generating a carrier signal having a carrier frequency f T , elements ( 12, 14 ) for generating pulses with a pulse repetition frequency f PW , elements ( 16 ) for distributing the carrier signal to a transmission branch and a receiving branch, elements ( 20 ) for modulate the carrier signal in the transmission path using the undelayed pulses, elements ( 22 ) for modulating the carrier signal in the receiving branch using the delayed pulses and for generating a reference signal, elements ( 24 ) for mixing the reference signal in the receiving branch with a received signal and elements ( 26 ) for integrating the mixed signal. Elements ( 28, 30 ) are provided for binary phase shift keying (BPSK) modulation of the carrier signal and elements ( 32 ) are provided for switching the polarity of the received signal. A method for suppressing interference in a radar device is also described.

[0001] The present invention relates to a radar device including meansfor generating a carrier signal with a carrier frequency f_(T), meansfor generating pulses with a pulse repetition frequency f_(PW), meansfor distributing the carrier signal to a transmission branch and areceiving branch, means for delaying the pulses, means for modulatingthe carrier signal in the transmission branch using the delayed pulsesand for generating a reference signal, means for mixing the referencesignal in the receiving branch with a received signal, and means forintegrating the mixed signal. The present invention also relates to amethod of suppressing the interference in a radar device including thesteps: generating a carrier signal having a carrier frequency f_(T),generating pulses having a pulse repetition frequency f_(PW),distributing the carrier signal to a transmission branch and a receivingbranch, generating the pulses, modulating the carrier signal in thetransmission branch using the undelayed pulses, modulating the carriersignal in the receiving branch using the delayed pulses and generating areference signal, mixing the reference signal in the receiving branchwith a received signal, and integrating the mixed received signal.

BACKGROUND INFORMATION

[0002] Radar devices and methods according to the related art are used,for example, in short-range sensing systems in motor vehicles. They areused, for example, to prevent accidents or to detect objects in a blindspot of a motor vehicle.

[0003]FIG. 1 shows a schematic view of the basic structure of a radardevice of the related art. A local oscillator (LO) 110 generates acarrier frequency f_(T). This carrier frequency is distributed by apower divider 116 to a transmission branch and a receiving branch. Inaddition to carrier frequency f_(T), a pulse generator 112 provides apulse repetition frequency f_(PW) to modulate the carrier frequency. Inthe transmission branch, this modulation occurs using switch 120, towhich the carrier frequency is applied and which is switched with thepulse repetition frequency. The signal thus generated is emitted by atransmitting antenna 136. A modulation also occurs in the receivingbranch. However, the pulses of the pulse repetition frequency aredelayed by a delaying device 118 for the purpose of this modulation.These delayed pulses are used to modulate carrier frequency f_(T) byoperating switch 122, to which the carrier frequency is also applied. Inthis way, a reference signal S_(R) is made available in the receivingbranch. This reference signal is mixed in a mixer 124 with a receivedsignal received via receiving antenna 134. The output signal of mixer124 is supplied to an integrating means 126, for example, a low-passfilter and an amplifier. The signal thus generated is supplied to asignal analyzer and controller 138, preferably after analog/digitalconversion. Signal analyzer and controller 138 now determines the delayof delaying device 118, which is varied between a value Δt_(min) andΔt_(max). For example, the delay may be varied by a microcontroller orby a digital signal processor. It is also conceivable that specialhardware is used for this purpose. If the transit time of the radarpulses, which as a rule is equal to twice the transit time between thetarget and antenna, is identical to the delay, the amplitude of theoutput signal of mixer 126 is at its maximum. A correlation receiver isthus available via which the distance to the target and the radial speedbetween the target and antenna may be inferred from the delay set bycontroller 138. By way of example, FIG. 1 shows only the formation ofthe in-phase (I) signal. The quadrature (Q) signal is formed in ananalogous manner by mixing with the carrier frequency, which is 90° outof phase.

[0004] It is basically desirable to suppress interference signalsoriginating from highly varied sources. The use of additional modulationof the microwave signal to separate the signal components reflected bythe targets from interference signals has already been described. Suchmethods in particular suppress interference by other uncodedtransmitters, broadcast transmitters for example, or noise.

[0005] However, radar devices are also subject to noise resulting fromparasitic effects which are essentially independent of the effect ofother radar sensors. Thus, for example, switches 120, 122 in FIG. 1 havein reality a finite ratio between the resistances in the off or oncondition R_(off)/R_(on). In addition, undesirable emissions or bridgingof the carrier frequency arise from the local oscillator, for example,to the reference input of the mixer. This means that an approximatelycontinuous leakage signal having the carrier frequency and low amplitudeis transmitted in the transmission pauses between the radar pulses. Thisleakage signal in particular is also present irrespective of the delayset in the reference branch and is mixed with the received signal. As aresult of this and other parasitic effects, an interference signal isreceived in addition from targets located outside the distance range(range gate) momentarily set by the delay in the reference signal. Ifsuch “undesirable” targets have a large backscattering cross-section orthey are within short range of the sensor, then the interference signalamplitude may be on the order of magnitude of the desired signalamplitude or exceed it and consequently result in measurement errors.

[0006] It is possible to improve the R_(off)/R_(on) ratio andaccordingly reduce the interference signal amplitude by using, forexample, several switches linked in series. However, this increases thetechnical complexity and consequently the costs.

ADVANTAGES OF THE INVENTION

[0007] According to a first embodiment, the present invention builds ona radar device of the related art by providing means for binary phaseshift keying (BPSK) modulation of the carrier signal. BPSK modulation ofthe carrier signal may be used to integrate interference signals withconstantly alternating signs in the subsequent integration while thedesired signal is integrated with a constant sign. The interferencesignals are suppressed in this manner.

[0008] According to a second embodiment, the present invention builds ona radar device of the related art by providing means to switch thepolarity of the received signal. In this manner, the subsequentintegration suppresses the interference signals to a great extent whilethe desired signals are further processed.

[0009] Preferably, means are provided for BPSK modulation of the carriersignal in the transmission branch. In this variant, the carrier signalin the receiving branch may be supplied to the mixer as a referencesignal without BPSK modulation. However, modulation takes place in thereceiving branch so that the information necessary for the interferencesignal suppression is present there.

[0010] However, it may also be advantageous to provide means for BPSKmodulation of the carrier signal in the receiving branch. In this case,a BPSK-modulated carrier signal is used as a reference signal while thetransmitted signal is transmitted unmodulated. The information necessaryfor the interference signal suppression is contained in the carriersignal in the receiving branch.

[0011] It is useful in particular if the BPSK modulation results in aswitchover of the phase angle for half a period T_(PW) of pulserepetition frequency f_(PW). In this way, the phase of the modulatedcarrier signal is switched between 0° and 180° after each half period.This periodic switchover of the phase angle advantageously ensures thatthe interference signals are integrated with a constantly alternatingsign while the desired signal is integrated with a constant sign.Referring to two periods in each case, a pulse is generated in thetransmission branch in each of the first and second half periods T_(PW)and in the receiving branch in each of the first and fourth halfperiods. The process is repeated after every two periods.

[0012] For effective interference signal suppression, it is advantageousin particular if the mixed signal is integrated over 2n periods T_(PW)of pulse repetition frequency f_(PW), n being an integer equal to 1, 2,3, . . . . This ensures that the interference signals are integratedalternately and accordingly suppressed.

[0013] It is useful if the ratio between carrier frequency f_(T) andpulse repetition frequency f_(PW) is an integer. This may be attained bydividing the carrier frequency by an integer. Another possibility forhaving the ratio as an integer is to generate the carrier frequency bymultiplying an oscillator frequency with an integer and to generate thepulse repetition frequency by dividing the same oscillator frequency byan integer. The ratio between the carrier frequency and the pulserepetition frequency being an integer provides an effective interferencesignal suppression since the start and end of the pulse always coincidewith a defined phase angle of the carrier signal.

[0014] It may also be advantageous to provide means for the BPSKmodulation of the carrier signal in the transmission branch and in thereceiving branch, to switch the phase angle in the receiving branch as aresult of the BPSK modulation for a period T_(PW) of pulse repetitionfrequency f_(PW) and to switch the phase angle in the transmissionbranch as a result of the BPSK modulation in every second pulse periodof pulse repetition frequency f_(PW) and in the transmission andreceiving branch for the length τ of each pulse. This makes it possibleto suppress even external interference signals in addition to theinterference signals based on parasitic effects.

[0015] Furthermore, it may be useful if switching means are provided toswitch the polarity of the received signal. Such hardware-based polarityswitching is suitable for ensuring the integration of the interferencesignal with an alternating sign.

[0016] However, it may also be useful if the polarity of the receivedsignal is switched digitally. Such digital and preferablyprogram-controlled polarity switching after analog/digital conversionreduces the hardware complexity. The integration in this case isexpediently digital, for example by decimation, i.e., low-pass filteringand subsequent reduction of the sampling rate. In this case, an externallow-pass is used to suppress aliasing. However, with this digitalmethod, the I signal or the Q signal must be sampled at a high bandwidthB (B>f_(PW)) and a correspondingly high sampling frequency and furtherprocessed digitally. This in turn requires additional hardwarecomplexity.

[0017] According to a first embodiment, the present invention builds onthe method of the related art in that binary phase shift keying (BPSK)modulation of the carrier signal occurs. BPSK modulation of the carriersignal may be used to integrate interference signals with a constantlyalternating sign in the subsequent integration while the desired signalis integrated with a constant sign. The interference signals aresuppressed in this manner.

[0018] According to a second embodiment, the present invention builds onthe method of the related art in that the polarity of the receivedsignal is reversed. In this manner, the subsequent integrationsuppresses the interference signals to a great extent while the desiredsignals are further processed.

[0019] Preferably, a BPSK modulation of the carrier signal occurs in thetransmission branch. In this variant, the carrier signal in thereceiving branch may be supplied to the mixer as a reference signalwithout BPSK modulation. However, a modulation takes place in thetransmission branch so that the information necessary for theinterference signal suppression is present there.

[0020] However, it may also be advantageous that a BPSK modulation ofthe carrier signal occurs in the receiving branch. In this case, aBPSK-modulated carrier signal is used as a reference signal while thetransmitted signal is transmitted unmodulated. The information necessaryfor interference signal suppression is contained in the carrier signalin the receiving branch.

[0021] It may also be useful if the BPSK modulation results in aswitchover of the phase angle for half a period T_(PW) of pulserepetition frequency f_(PW). In this way, the phase of the modulatedcarrier signal is switched between 0° and 180° after each half period.This periodic switchover of the phase angle advantageously ensures thatthe interference signals are integrated with a constantly alternatingsign while the desired signal is integrated with a constant sign.

[0022] Preferably, the mixed signal is integrated over 2n periodsT_(PW), n being an integer equal to 1, 2, 3, . . . of pulse repetitionfrequency f_(PW). This ensures that the interference signals areintegrated alternately and thus suppressed.

[0023] It is useful that the ratio between carrier frequency f_(T) andpulse repetition frequency f_(PW) is an integer. This may be attained bydividing the carrier frequency by an integer. Another possibility forhaving an integer ratio is to generate the carrier frequency bymultiplying an oscillator frequency with an integer and to generate thepulse repetition frequency by dividing the same oscillator frequency byan integer. The ratio between the carrier frequency and the pulserepetition frequency being an integer provides an effective interferencesignal suppression since the start and end of the pulse always coincidewith a defined phase angle of the carrier signal.

[0024] Also, it may be advantageous if a BPSK modulation of the carriersignal occurs in the transmission branch and in the receiving branch, ifthe phase angle is switched in the receiving branch as a result of theBPSK modulation for a period of pulse repetition frequency f_(PW) and ifthe phase angle is switched in the transmission branch as a result ofthe BPSK modulation in every second pulse period of pulse repetitionfrequency f_(PW) and in the transmission and receiving branch for thelength τ of each pulse. This makes it possible to suppress even externalinterference signals in addition to the interference signals based onparasitic effects. In this embodiment, one pulse is generated in thetransmission branch and one in the receiving branch in each periodT_(PW) of pulse repetition frequency f_(PW).

[0025] Preferably the polarity of the received signal is switched byswitching means. Such hardware-based polarity switching is suitable toensure the integration of the interference signal with an alternatingsign.

[0026] It may also be advantageous if the polarity of the receivedsignal is switched digitally. Such digital and preferablyprogram-controlled polarity switching after analog/digital conversionreduces the hardware complexity. Integration in this case is expedientlydigital, for example by decimation, i.e., low-pass filtering andsubsequent reduction of the sampling rate. In this case, an externallow-pass is used to suppress aliasing. However, with this digitalmethod, the I signal or the Q signal must be sampled at a high bandwidthB (B>f_(PW)) and a correspondingly high sampling frequency and furtherprocessed digitally. This in turn requires additional hardwarecomplexity.

[0027] The present invention is based on the surprising knowledge thatit is possible to suppress interference by parasitic effects in radardevices actually constructed with relatively little technicalcomplexity. The use of a BPSK modulation or switching the polarity ofthe received signal makes it possible to integrate the interferencesignals with constantly alternating signs while the desired signal isintegrated with a constant sign. As an advantageous embodiment inparticular, it should be mentioned that it is not only possible tosuppress interference caused by parasitic effects in both thetransmission branch and the receiving branch, but rather it is alsopossible to suppress external interference signals.

DRAWINGS

[0028] An example of the present invention will now be explained basedon preferred embodiments with reference to the appended drawing inwhich:

[0029]FIG. 1 shows a schematic representation of a radar device of therelated art;

[0030]FIG. 2 shows a schematic representation of a radar deviceaccording to the present invention;

[0031]FIG. 3 shows graphic representations of the amplitudes oftransmitted and received signals of the radar device according to FIG.2;

[0032]FIG. 4 shows a schematic representation of a radar deviceaccording to the present invention;

[0033]FIG. 5 shows schematic representations of the amplitudes oftransmitted and received signals of the radar device according to FIG.4;

[0034]FIG. 6 shows a schematic representation of a radar deviceaccording to the present invention;

[0035]FIG. 7 shows a schematic representation of a radar deviceaccording to the present invention and

[0036]FIG. 8 shows schematic representations of the amplitudes oftransmitted and received signals of the radar device according to FIG.7.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0037]FIG. 2 shows a schematic representation of a radar deviceaccording to the present invention. A local oscillator 10 is connectedto a power divider 16. This power divider 16 is connected to atransmission branch. One portion of the power of local oscillator 10 isdecoupled by power divider 16 into the receiving branch. In addition,local oscillator 10 is connected to a frequency divider 12. Frequencydivider 12 reduces carrier frequency f_(T) of local oscillator 10 by thefactor N. This reduced frequency is supplied to a pulse shaper 40 and adelay device 18. This pulse shaper 40 operates a switch 20 in thetransmission branch to modulate the carrier signal. The delayed outputsignal of delay device 18 is sent to a pulse shaper 42. This pulseshaper 42 generates pulses with which the carrier signal in thereceiving branch is modulated using switch 22. A reference signal S_(R)is generated in this manner. Thus signals are present in both thetransmission branch and in the receiving branch that are modulated withthe pulses of the respective pulse shapers 40, 42. The signal in thetransmission branch is subsequently supplied to a BPSK modulation 28,which switches the transmitted signal between 0° and 180°. Thisswitchover between 0° and 180° occurs within half a period, whichcorresponds to period length T_(PW) of doubled pulse repetitionfrequency F_(PW). This is accomplished in that BPSK modulation 28 iscontrolled by an output signal of frequency divider 12. In this manner,the phase is switched after every half period of pulse repetitionfrequency f_(PW). The BPSK signal thus modulated is transmitted bytransmitting antenna 36 and received by receiving antenna 34 after beingreflected by a target. The signal received by receiving antenna 34 issupplied to a mixer 24 where it is mixed with reference signal S_(R).The output signal of mixer 24 is integrated and amplified in a low-passfilter 26. The output signal of low-pass filter 26 is sent to a signalanalysis unit and controller 38. Controller 38 sets the delay of delaydevice 18. The result of the periodic switchover of the phase angle isthat the corresponding interference signal is integrated with aconstantly alternating sign and the interference signal is thusminimized. However, the desired signal is integrated with a constantsign. For effective interference signal suppression, it is alsonecessary that the carrier frequency f_(T) to pulse repetition frequencyf_(PW) ratio be an integer: N=f_(T)/f_(PW). This may be accomplished, asshown, by dividing the carrier frequency by the factor N or bymultiplying an oscillator frequency f_(O) with f_(T)=N₁f_(O) anddividing f_(PW)=f_(O)/N₂ by N₁N₂=N and N₂=1, 2, . . . (not shown). Theformation of the in-phase (I) signal is shown in FIG. 2. The quadrature(Q) signal is formed by analogy by mixing with carrier frequency f_(T),which has been phase shifted by 90°.

[0038] The amplitude of transmitted signal S_(T) is shown over twoperiods T_(PW) of pulse repetition frequency f_(PW) in the upper portionof FIG. 3. Reference signal S_(R) is shown over the same time span inthe lower portion of FIG. 3. FIG. 3 shows the signal relationships thatoccur in a circuit according to FIG. 2, i.e., with a BPSK modulation inthe transmission branch. It may be seen that the transmitted signal isswitched over after every period T_(PW) of pulse repetition frequencyf_(PW). The delayed received signal is not phase-modulated.

[0039] Another radar device is again illustrated by way of example inFIG. 4 using the in-phase (I) signal. In this case also, the quadrature(Q) signal is formed in an analogous manner by mixing with carrierfrequency f_(T), which is phase-shifted by 90°. Components correspondingto the same components in FIG. 2 are identified by the same referencesymbols. In contrast to FIG. 2, the receiving branch is BPSK-modulatedin the radar device according to FIG. 4. However, the transmittedsignals are transmitted by transmitting antenna 36 without prior BPSKmodulation. The BPSK modulation in the receiving branch generates aBPSK-modulated reference signal S_(R) which is mixed with the receivedsignal in mixer 24.

[0040] The waveform of the signals that occur in a circuit according toFIG. 4 is shown graphically in FIG. 5. Transmitted signal S_(T) isplotted in the upper portion of FIG. 5. Reference signal S_(R) is shownin the lower portion of FIG. 5. Based on FIG. 5, it may be seen that thetransmitted signal is not phase-modulated. The phase of the delayedreceived signal is switched over after each half period T_(PW) of thepulse repetition frequency.

[0041] Another embodiment of a radar device of the present invention isshown in FIG. 6. Again, components that correspond to those of FIG. 2are identified with the same reference symbols. The special feature inFIG. 6 is that means 32 are provided instead of BPSK modulation forswitching the polarity of the received signal. This also makes itpossible to suppress the interference based on the subsequentintegration since the interference is integrated with an alternatingsign while the desired signals are always integrated with the same sign.

[0042] An additional embodiment of a radar device of the presentinvention is shown schematically in FIG. 7. In this case, both thetransmission branch as well as the receiving branch are provided withBPSK modulation 28, 30. BPSK modulation 30 occurs in the receivingbranch as a function of the output signal of an OR gate 44. Inputsignals of this OR gate 44 are the delayed pulses as well as pulserepetition frequency f_(PW) divided by two. In this manner, the phasemodulation in the receiving branch occurs during length τ of the pulsesand in every second pulse period T_(PW) of the pulse repetitionfrequency. This, however, requires greater hardware complexity.

[0043] BPSK modulation 28 in the transmission branch occurs as afunction of an additional OR gate 45. Input signals of this OR gate 45are the pulses from pulse shaper 40 and pulse repetition frequencyf_(PW) divided by four at the output of an additional divider 46. Thephase modulation thus occurs during length τ of the pulses and for thelength of two periods each of pulse repetition frequency f_(PW) in thethird and fourth periods of four periods. After every four periods, themodulation pattern just described is repeated in a similar manner.

[0044] The signals that occur in a radar device of the present inventionaccording to FIG. 7 are shown in FIG. 8. Transmitted signal S_(T) isagain shown in the upper portion of FIG. 8. Reference signal S_(R) isshown in the lower part. It may be seen that the phase of thetransmitted signal is switched over after every second period T_(PW) ofthe pulse repetition frequency and during length τ of each pulse. Thephase of the reference signal is, however, switched after each periodT_(PW) of the pulse repetition frequency and during length τ of eachpulse.

[0045] The above description of the exemplary embodiments according tothe present invention is only intended to illustrate and not to limitthe invention. Various changes and modifications are possible within thescope of the invention without departing from the scope of the inventionand its equivalents.

What is claimed is:
 1. A radar device comprising means (10) forgenerating a carrier signal having a carrier frequency f_(T), means (12)for generating pulses having a pulse repetition frequency f_(PW), means(16) for distributing the carrier signal to a transmission branch and areceiving branch, means (18) for delaying the pulses, means (20) formodulating the carrier signal in the transmission branch using theundelayed pulses, means (22) for modulating the carrier signal in thereceiving branch using the delayed pulses and for generating a referencesignal, means (24) for mixing the reference signal in the receivingbranch with a received signal and means (26) for integrating the mixedsignal, wherein means (28, 30) are provided for binary phase shiftkeying (BPSK) modulation of the carrier signal.
 2. A radar devicecomprising means (10) for generating a carrier signal having a carrierfrequency f_(T), means (12) for generating pulses having a pulserepetition frequency f_(PW), means (16) for distributing the carriersignal to a transmission branch and a receiving branch, means (18) fordelaying the pulses, means (20) for modulating the carrier signal in thetransmission branch using the undelayed pulses, means (22) formodulating the carrier signal in the receiving branch using the delayedpulses and for generating a reference signal, means (24) for mixing thereference signal in the receiving branch with a received signal andmeans (26) for integrating the mixed signal, wherein means (32) areprovided for switching the polarity of the received signal.
 3. The radardevice as recited in claim 1 or 2, wherein means (28) are provided forBPSK modulation of the carrier signal in the transmission branch.
 4. Theradar device as recited in one of the preceding claims, wherein means(30) are provided for BPSK modulation of the carrier signal in thereceiving branch.
 5. The radar device as recited in one of the precedingclaims, wherein the phase angle is switched over by the BPSK modulationfor half a period T_(PW) of the pulse repetition frequency f_(PW). 6.The radar device as recited in one of the preceding claims, wherein themixed signal is integrated over an integral number of 2n periods T_(PW)of pulse repetition frequency f_(PW) where n=1, 2, 3, . . . .
 7. Theradar device as recited in one of the preceding claims, wherein,referring to two pulse periods T_(PW) in each case, a pulse is generatedin the transmission branch in each of the first and second half pulseperiods and in the receiving branch in each of the first and third halfpulse periods and the process is repeated after every two pulse periodsT_(PW).
 8. The radar device as recited in one of the preceding claims,wherein the ratio of the carrier frequency f_(T) to the pulse repetitionfrequency f_(PW) is an integer. 9 The radar device as recited in one ofthe preceding claims, wherein means (28, 30) are provided for BPSKmodulation of the carrier signal in the transmission branch and in thereceiving branch, the phase angle is switched in the transmission branchas a result of the BPSK modulation for two periods T_(PW) each of thepulse repetition frequency f_(PW) and during the length τ of each pulse,and the phase angle is switched in the receiving branch as a result ofthe BPSK modulation in every pulse period of the pulse repetitionfrequency f_(PW) and during the length τ of each pulse.
 10. The radardevice as recited in one of the preceding claims, wherein the mixedsignal is integrated over an integral number of 4n periods T_(PW) ofpulse repetition frequency f_(PW), where n=1, 2, 3, . . . .
 11. Theradar device as recited in one of the preceding claims, wherein onepulse each is generated in the transmission and receiving branch in eachpulse period T_(PW).
 12. The radar device as recited in one of thepreceding claims, wherein switching means (32) are provided to switchthe polarity of the receiving signal.
 13. The radar device as recited inone of the preceding claims, wherein the polarity of the received signalis switched digitally.
 14. A method for suppressing interference in aradar device comprising the steps generation of a carrier signal havinga carrier frequency f_(T), generation of pulses having a pulserepetition frequency f_(PW), distribution of the carrier signal to atransmission branch and a receiving branch, delay of the pulses,modulation of the carrier signal in the transmission branch using theundelayed pulses, modulation of the carrier signal in the receivingbranch using the delayed pulses and generation of a reference signal,mixing the reference signal in the receiving branch with a receivedsignal and integration of the mixed signal, wherein the carrier signalis modulated by binary phase shift keying (BPSK) modulation.
 15. Amethod for suppressing interference in a radar device comprising thesteps generation of a carrier signal having a carrier frequency f_(T),generation of pulses having a pulse repetition frequency f_(PW),distribution of the carrier signal to a transmission branch and areceiving branch, delay of the pulses, modulation of the carrier signalin the transmission branch using the undelayed pulses, modulation of thecarrier signal in the receiving branch using the delayed pulses andgeneration of a reference signal, mixing the reference signal in thereceiving branch with a received signal and integration of the mixedsignal, wherein the polarity of the received signal is switched over.16. The method as recited in claim 14 or 15, wherein a BPSK modulationof the carrier signal occurs in the transmission branch.
 17. The methodas recited in one of claims 14 through 16, wherein a BPSK modulation ofthe carrier signal occurs in the receiving branch.
 18. The method asrecited in one of claims 14 through 17, wherein the phase angle isswitched over by the BPSK modulation for half a period T_(PW) of pulserepetition frequency f_(PW).
 19. The method as recited in one of claims14 through 18, wherein the mixed signal is integrated over an integralnumber of 2n periods T_(PW) of pulse repetition frequency f_(PW), wheren=to 1, 2, 3, . . . .
 20. The method as recited in one of claims 14through 19, wherein the ratio of carrier frequency f to the pulserepetition frequency f_(PW) is an integer.
 21. The method as recited inone of claims 14 through 20, wherein a BPSK modulation of the carriersignal occurs in the transmission branch and in the receiving branch,the phase angle is switched in the transmission branch as a result ofthe BPSK modulation for two periods of the pulse repetition frequencyf_(PW) and during the length τ of each pulse and the phase angle isswitched in the receiving branch as a result of the BPSK modulation inevery second pulse period of pulse repetition frequency f_(PW) andduring the length τ of each pulse.
 22. The method as recited in one ofclaims 14 through 21, wherein the polarity of the received signal isswitched over by switching means (32).
 23. The method as recited in oneof claims 14 through 22, wherein the polarity of the received signal isswitched over digitally.